A typical gas turbine engine includes a turbine module with one or more turbines for extracting energy from a stream of working medium fluid. Each turbine has a hub capable of rotation about an engine axis. The hub includes peripheral slots for holding one or more arrays (i.e. rows) of blades. Each blade includes an attachment adapted to fit in one of the slots, a platform and an airfoil. When the blades are installed in the hub the platforms cooperate with each other to partially define the radially inner boundary of an annular working medium flowpath. The airfoils span across the flowpath so that the airfoil tips are in close proximity to a nonrotatable casing. The casing circumscribes the blade array to partially define the radially outer boundary of the flowpath. Alternatively, a blade may have a radially outer platform or shroud that partially defines the radially outer boundary of the flowpath. The radially inner platform and the radially outer platform (if present) partially define flowpath endwalls.
A typical turbine module also includes one or more arrays of vanes that are nonrotatable about the engine axis. Each vane has radially inner and outer platforms that partially define the radially inner and outer flowpath boundaries. An airfoil spans across the flowpath from the inner platform to the outer platform. The vane platforms partially define the flowpath endwalls.
During engine operation, a stream of working medium fluid flows through the turbine flowpath. Near the endwalls, the fluid flow is dominated by a vortical flow structure known as a horseshoe vortex. The vortex forms as a result of the endwall boundary layer, which separates from the endwall as the fluid approaches the leading edges of the airfoils. The separated fluid reorganizes into the horseshoe vortex. There is a high loss of efficiency associated with the vortex. The loss is referred to as “secondary” or “endwall” loss. As much as 30% of the loss in a row of airfoils can be attributed to endwall loss. Further description of the horseshoe vortex, the associated fluid dynamic phenomena and geometries for reducing endwall losses can be found in U.S. Pat. No. 6,283,713 entitled “Bladed Ducting for Turbomachinery” and in Sauer et al., “Reduction of Secondary Flow Losses in Turbine Cascades by Leading Edge Modifications at the Endwall”, ASME 2000-GT-0473.
Notwithstanding the presumed merits of the geometries disclosed in the above references, other ways of mitigating secondary flow losses are sought.